1. Field of the Invention
The present invention relates to a polymeric sensor having selective binding affinity for an analyte which is indicative of food spoilage. More particularly, this invention is directed to a polymer containing incorporated therein a macrocyclic transition metal complex which selectively binds a biogenic amine, whereupon the polymer undergoes a detectable color change and thereby indicates the extent of food spoilage. Selectivity of the polymer sensor may be enhanced through the process of molecular imprinting.
2. Description of the Related Art
Polymeric sensors are prepared by ionically or covalently attaching an atomic, ionic, or molecular site to a polymer that, upon association with a particular analyte, will exhibit a detectable change in a measurable physical property. Exemplary measurable physical properties include spectroscopic (i.e., electronic absorbance or luminescence), electrochemical, or magnetic properties. The change in the physical property then provides a probe for the presence or absence of the associated analyte that can be measured using appropriate instrumentation or by direct observation. The polymer provides a support matrix that serves to immobilize the sensor sites and provide a localized density of the sensor sites as a means of optimizing detection of the analyte.
Molecular imprinting essentially involves making a polymer cast of a target molecule. The process of making the polymer cast involves dissolving the target molecule to be imprinted in a suitable solvent. Normally, a co-monomer, cross-linking monomer and a polymerization initiator are added to the reaction mixture. Radiation (photochemical or ionizing) or thermal energy is then applied to the reaction mixture to drive the polymerization process, ultimately resulting in the formation of a solid polymer. The resulting polymer may be processed using conventional polymer processing technologies, assuming those processes do not alter the structure of the molecularly imprinted sites. The imprinted molecule is extracted using methods appropriate for dissociating the target molecule from the polymer. Details of target molecule dissociation from the polymer are dependent upon the nature of the chemical interaction between the target molecule and the polymer binding site. The polymer dissociated from the target molecule possesses binding sites optimized for the structural and electronic properties of the target molecule. Mosbach U.S. Pat. No. 5,110,833 describes the preparation of synthetic enzymes and synthetic antibodies by molecular imprinting techniques.
The advantages to polymer sensors, including molecularly imprinted polymer (MIP) sensors, include their high molecular sensitivity, robustness, long storage life without losing their sensitivity, low cost, and ease of production. It is for these reasons that MIPs are being utilized in many industries. Current biomedical applications include drug monitoring devices and implants to monitor glucose levels.
Tetraazacyclotetradecanenickel(II), referred to as Ni(cyclam)2+, due to its geometric properties and resulting large ligand field stabilization energy, exists in water predominantly as a square planar, four-coordinate, yellow-colored complex (Connolly, P. J.; Billo, E. J. Inorg. Chem. 1987, 26, 3224-3226). Upon addition of ethylene diamine, aqueous solutions of Ni(cyclam)2+ turn from yellow to a blue/violet color, the origin of which has been attributed to cis coordination of the diamine to two adjacent coordination sites of the central nickel(II)ion (Billo, E. J. Inorg. Chem. 1984 23, 2223-2227). The availability of the cis coordination sites on the nickel(II) ion is made possible by folding of the cyclam ligand, a process that is known to involve isomerization of the nitrogen donor set (Barefield, E. K.; Bianchi, A.; Billo, E. J.; Connolly, P. J.; Paoletti, P.; Summers, J. S.; Van Derveer, D. G. Inorg. Chem. 1986, 25, 4197-4202). The change in geometry from four-coordinate in the absence of the diamine to six-coordinate in the presence of the diamine results in a change in the ligand field splitting of the central nickel(II) ion, and the blue/violet color of the product. See FIG. 1.
A Ni(II) complex related to Ni(cyclam)2+, 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecanenickel(II), referred to as Ni(Me2[14]aneN6)2+, possesses a 14-membered skeleton structure, although the propylene bridges of cyclam are now replaced with tertiary amines. This compound is known to exist predominantly in the yellow form in aqueous solution and has properties similar to those of Ni(cyclam)2+ (Suh, M. P.; Kang, S.-G. Inorg. Chem. 1988, 27, 2544-2546). A related compound, with 2-hydroxyethyl groups at the 1 and 8 positions in place of the methyl groups, has recently been found to react with ethylene diamine to yield a cis-hexacoordinate nickel(II) complex (Xiang, H.; Lu, T.-B; Chen, S.; Mao, Z.-W; Feng, X.-L; Yu, K.-B. Polyhedron 2001, 20, 313-319).
Meat spoilage occurs as bacteria begin to grow unchecked following respiration and circulation cessation at the time of slaughter. One of the markers of meat spoilage is the decarboxylation of free amino acids on and in the meat by enzymes released by spoilage microorganisms. Two of these products, putrescine and cadaverine, are particularly distinctive in odor, correlate well with surface bacterial counts, and are widely used to evaluate meat freshness both by trained meat inspectors and the individual consumers. Another product, histamine, is of interest due to its apparent ability to potentiate histamine intoxication, a form of food poisoning associated with the consumption of spoiled fish.
Accordingly, there is a need for a robust sensor device which accurately, simply and rapidly detects the presence of biogenic amines in food products.